RELATIONSHIP BETWEEN NUCLEOSIDE DIPHOSPHATE KINASE ACTIVITY AND LIGHT‐LIMITED GROWTH RATE IN THE MARINE DIATOM THALASSIOSIRA PSEUDONANA (BACILLARIOPHYCEAE)1

Abstract

ABSTRACTLight‐limited cultures of the marine diatom Thalassiosira pseudonana (Hustedt) Hasle and Heimdal (3H clone) were grown over a range of growth rates between 0.06 and 1.64 d−1. Variations in cell volume, cell quotas of carbon, nitrogen, and protein, and maximal activity of the enzyme nucleoside diphosphate kinase (NDPK) were measured and examined as a function of growth rate. NDPK from T. pseudonana showed Km values of 0.24 and 0.68 mM for thymidine 5′‐diphosphate and adenosine 5′‐triphosphate (ATP), respectively, which are similar to those found for NDPK from a variety of organisms, from bacteria to mammals. An apparent activation enthalpy of 3.52 kCal·mol−1 was determined from Arrhenius plots. No thermodynamic transition points were noted over a temperature range from 10° to 25°C. NDPK activity was significantly correlated with growth rate but not with cell volume, carbon, nitrogen, or protein; for interspecific comparisons, normalization of enzyme activity to cell number may be most meaningful. NDPK activity per cell versus growth rate followed a U‐shaped relationship, being relatively constant between 0.5 and 1.0 d−1 and rising at higher and lower growth rates. Over this range, enzyme activity may be regulated by substrate concentration (ATP or other nucleoside triphosphates) or by adenylate energy charge. At higher growth rates where energy charge and substrate concentrations are probably high, changes in enzyme concentration appear to be required. The reasons for a rise in enzyme activity at low growth rate is unclear. Simultaneous measurement of nucleoside di‐ and triphosphate levels alongside NDPK measurements may help clarify the relationship, but these preliminary experiments indicate that NDPK is of limited usefulness as an index of in situ growth rate.